Website Notes: Atomic Physics.
1. So far we are familiar with three atomic Models. The first was that of
Democritus (460-370) BC, the ancient Greek philosopher, who proposed
that the smallest unit of matter was a small, single, indivisible particle,
2. This belief lasted for almost 2300 years, until the 1890's when the
British Physicist, J. J. Thomson discovered the electron using his CRT.
His model was known as the "Plum Pudding" Model.
3. In the early 1900's, Ernest Rutherford, also from England proposed his
Planetary Model, based on his "Gold Foil" Experiment. This atomic model
was characterized as mostly empty space, but with a very dense, positively
charged nucleus, and orbiting electrons.
4. While the Rutherford (Planetary) model focused on describing the nucleus,
the electron was depicted as an orbiting planet. The flaw with the planet-like
model is that an electron particle moving in a circular path would be
5. An accelerating electron creates a changing magnetic field. This changing
magnetic field would carry energy away from the electron, eventually slowing
it down and allowing it to be "captured" by the nucleus.
6. In 1913, the Danish physicist Niels Bohr (1885-1962) managed to
explain the a new atomic model as an extension of Rutherford's
description of the atom.
7. Bohr agreed that the negatively charged electrons revolve about the
positively charged atomic nucleus because of the attractive electrostatic
force according to Coulomb's law.
8. But the electron can be taken not only as a particle, but also as a
de Broglie wave (wave of matter) which interferes with itself.
9. The orbit is only stable, if it meets the condition for a standing wave:
The circumference must be an integer multiple of the wavelength.
The consequence is that only special values of radius and energy are allowed.
10. According to classical electrodynamics, a charge, which is subject to
centripetal acceleration on a circular orbit, should continuously radiate
11. Thus, because of the loss of energy, the electron should spiral into the
nucleus very soon. By contrast, an electron in Bohr's model emits no energy,
as long as its energy has one of the above-mentioned values.
12. However, an electron which is not in the lowest energy level (n = 1), can
make a spontaneous change to a lower state and thereby emit the energy
difference in the form of a photon (particle of light).
13. By calculating the wavelengths of the corresponding electromagnetic
waves, one will get the same results as by measuring the lines of the
14. We must not take the idea of electrons, orbiting around the atomic
nucleus, for reality. Bohr's model of the hydrogen atom was only an
intermediate step on the way to a precise theory of the atomic structure,
which was made possible by quantum mechanics and quantum
15. Still, the most important properties of atomic and molecular structure
may be exemplified using a simplified picture of an atom that is called the
Planetary Quantum or Bohr Model.
16. Again, this model was proposed by Niels Bohr in 1915 and although it
is not completely correct, but it has many features that are approximately
correct and it is sufficient for much of our discussion.
17. The correct theory of the atom is called Quantum Mechanics; the Bohr
Model is an approximation to quantum mechanics that has the virtue of
being much simpler to understand.
18. In the Bohr Model the neutrons and protons occupy a dense central
region called the nucleus, and the electrons orbit the nucleus much like
planets orbiting the Sun (but the orbits are not confined to a plane as is
approximately true in the Solar System).
19. This similarity between a Planetary Model and the Bohr Model of the
atom ultimately arises because the attractive gravitational force in a solar
system and the attractive Coulomb (electrical) force between the positively
charged nucleus and the negatively charged electrons in an atom are
mathematically of the same form.
20. The form is the same, but the intrinsic strength of the Coulomb interaction
is much larger than that of the gravitational interaction; in addition, there are
positive and negative electrical charges so the Coulomb interaction can be
either attractive or repulsive, but gravitation is always attractive in our present
21. Based on the spectrum of atomic hydrogen and the fact that each gas has
a unique emission and absorption spectrum, Niels Bohr proposed his Quantum-
Mechanical Atomic Model instead of Rutherford's Planetary Model.
22. He proposed that electrons can move from one energy level to another by
absorbing or emitting photons. His equations were: (a) for orbital radii.
rn = 5.3x10-11 m x n2 , and (b) for the ionization energy associated with each
level,En = -13.6 eV x 1/n2 , with n = 1,2,3,... , the energy level number.
The electron-volt (eV) is the energy unit for electrons, 1 eV = 1.6x10-19 J .
23. Werner Heisenberg (1901-1976) determined that it is not possible to know
the exact position and momentum of the electron, the Uncertainty Principle.
24. Arthur Holly Compton (1892-1962) bombarded a graphite block with X-rays
demonstrating the momentum of photons (The Compton Effect ). The equation
is mv = p = h/λ .
25. James Chadwick (1891-1974) an original member of Rutherford's research
team proved the existence of neutrons in 1932.
26. Light Amplification by Stimulated Emission of Radiation (LASER), which
was explained by Einstein in 1917, was invented in 1960. Laser light is very
directional, powerful, monochromatic, and coherent, making it very useful.
Website Notes: Nuclear Physics.
1. Henri Becquerel (1852-1908) accidentally found that all compounds
containing uranium emitted rays that penetrate and fog photographic plates,
after examining a mysterious rock.
2. Ernest Rutherford (1871-1937) identified alpha, beta, and gamma radiation
and used alpha particles to bombard gold foil. He found that most of an atom
is empty space but contains a massive positively charged nucleus.
3. The Curies, Pierre and Marie, were the first to discover other radioactive
elements, for example, Polonium and Radium.
4. The nucleus can be characterized by a mass number, A, an atomic number,
Z, and a neutron number, N, with A = Z + N. Atoms having the same number
of protons but different amounts of neutrons are called isotopes.
5. The nucleus of an atom contains most of the mass, consists of protons
and neutrons, with protons and neutrons termed as "nucleons."
6. We use the Atomic Mass Unit (amu), or u, for nucleon mass. To convert just
use the fact that 1 u = 1.6605x10-27 kg. This means that we now have the
mass of a proton as, 1 p = 1.007825 u, and a neutron, 1 n = 1.008665 u.
7. The change, transmutation, in an atomic nucleus can be natural or artificial.
Enrico Fermi (1901-1954) successfully produced artificially radioactive elements
in the laboratory.
8. Radioactive decay produces three kinds of particles: alpha, α, helium nuclei;
beta, β, high-speed electrons; and gamma, γ, ray photons.
9. Bombardment of nuclei by protons, neutrons, alpha particles, electrons,
gamma rays, or other nuclei can produce a nuclear reaction.
10. Linear accelerators, synchrotrons, and super-colliders produce high-energy
protons and electrons which can collide with each other or an atomic nucleus.
11. Particle detectors include photographic plates, the Geiger-Muller tube,
scintillation screens, and the cloud chamber.
12. Alpha can be stopped by thick paper, beta by thick aluminum foil, and a
few centimeters of lead will stop gamma.
13. During positron decay a proton changes into a neutron with the emission
of a positron and a neutrino.
14. When matter and antimatter combine, all matter is converted into energy,
or lighter matter-antimatter particle pairs. By pair production, energy is
converted into a matter-antimatter particle pair.
15. The weak interaction operates in beta decay while the strong force binds
the nucleus together. During beta decay a neutron changes into a proton and
the nucleus emits a beta particle and a mass-less antineutrino.
16. The binding energy is the energy equivalent of the mass defect. The
assembled nucleus has less mass than its constituent parts due to mass-to-
energy conversion, Binding Energy = (Δm)c2 , with Δm as the mass defect.
17. Nuclear reactors use the energy released in fission as heat to boil water,
which produces steam, that turns turbine blades to run a generator.
18. The binding energy of the nucleus is the difference in energy between its
nucleons when bound and its nucleons when unbound. Energy-mass equivalence
can be computed using 1 amu = 931 MeV.
19. The half-life, T½ , is the time required for half the original nuclei of a
radioactive substance to undergo radioactive decay. We use the equation
A = A0∙2-n where n is the number of half-lives, and A indicating amount.
20. The decay constant, lambda, λ, indicates the rate of radioactive decay.
Half-life can also be calculated by T½ = .693/λ .
21. Nuclear reactions involve a change in the nucleus and can be given by
equations. In equations for nuclear reactions, subscripts and superscripts must
agree on both sides.
22. In a nuclear equation the sums of the subscripts (atomic number or particle
charge) on both sides of the equation are equal and the sums of the superscripts
(mass number) on both sides of the equation are equal.
23. In fission, heavier nuclei split to form lighter nuclei and energy is released.
In fusion, lighter nuclei combine to form heavier nuclei with more binding energy.
Website Notes: Particle Physics.
1. Chemistry can be understood in the physics of 3 particles (proton, neutron
and electron), and the influence of the electromagnetic force.
2. Nuclear physics can be understood in the physics of 4 particles (proton,
neutron, electron and electron neutrino), and the influence of the strong and
weak nuclear forces together with the electromagnetic force.
3. The Standard Model Theory (SM) of particle physics provides a framework for
explaining chemistry and nuclear physics (low energy processes). It additionally
provides an explanation for sub-nuclear physics and some aspects of cosmology
in the earliest moments of the universe (high energy processes).
4. Physicists currently believe there are three types of basic building blocks of
matter (quarks, leptons, bosons). Quarks and leptons make up matter, which
is held together by bosons. Each boson is associated with a force.
5. The photon, the unit of the electromagnetic force, holds the electron to the
nucleus in the atom. The way these particles combine dictates the structure of
HERE for Particle descriptions.
Click HERE for Particle descriptions.
6. The Standard Model is conceptually simple and contains a description of the
elementary particles and forces. The SM particles are the 6 quarks, which include
the up and down quarks that make up the neutron and proton.
7. The 6 leptons include the electron and its partner, the electron neutrino. The 4
bosons are particles that transmit forces and include the photon, which transmits
the electromagnetic force. Click HERE for Force descriptions.
8. With the observation of the tau neutrino at Fermilab, all 12 fermions and all 4
gauge bosons have been observed. Seven of these 16 particles (charm, bottom,
top, tau neutrino, W, Z, gluon) were predicted by the Standard Model before
they were observed experimentally!
9. There is one additional particle predicted by the Standard Model called the
Higgs, which has not yet been observed. It is needed in the model to give mass
to the W and Z bosons, consistent with experimental observations.
10. While photons and gluons have no mass, the W and Z are quite heavy.
The W weighs 80.3 GeV (80 times as much as the proton) and the Z weighs
11. The Higgs is expected to be heavy as well. Direct searches for it at CERN
dictate that it must be heavier than 110 GeV.
And to get full credit for homework make sure you follow these steps:
(i) read the problem and identify the given variables
(ii) determine what you are asked to solve for
(iii) find the correct formula to use
(iv) use algebra to isolate the unknown
(v) substitute-in the given information and simplify.
Problem Set #2 (Test Review KEY).
For the Lab Abstract template. Click HERE.